Halogen lamps and High Intensity Discharge (HID) lamps are conventional mainstream of light sources for vehicle lamps, particularly, for headlamps. In recent years, vehicle lamps are developed to use semiconductor light sources, such as Light Emitting Diodes (LEDs) and Laser Diodes (LDs), instead of these halogen lamps and HID lamps (for example, JP-A-2006-103404).
A vehicle lamp includes a plurality of light sources which are configured to be lit on and off individually. For example, a vehicle lamp includes a low beam light source and a high beam light source. FIGS. 1A and 1B are circuit diagrams of a vehicle lamp including a plurality of light sources which are investigated by the inventors. In FIGS. 1A and 1B, a first light source 302 is for low beam and a second light source 304 is for high beam.
In FIG. 1A, the first light source 302 and the second light source 304 of a vehicle lamp 300U are configured as individual light emitting units and are respectively connected to a lighting circuit 400U via wire harnesses 306, 308. The first light source 302, the second light source 304, the wire harnesses 306, 308, and a wire 416 in the lighting circuit 400U form a series current path.
The lighting circuit 400U includes a drive circuit 414, a bypass switch 430, and a switch driver 432. The drive circuit 414 is configured by combining (i) a constant current converter or (ii) a constant voltage converter, with a constant current circuit.
When a power-supply voltage VIN is supplied to an LO terminal, the drive circuit 414 supplies a drive current ILAMP (lamp current) to a current path including the first light source 302 and the second light source 304.
The bypass switch 430 is provided in parallel with the second light source 304, and the switch driver 432 turns off the bypass switch 430 when an HI terminal is at a high level. In this case, the drive current ILAMP is supplied to the second light source 304 such that the second light source 304 is lit on. The switch driver 432 turns on the bypass switch 430 when the HI terminal is at a low level. In this case, the drive current ILAMP flows through the bypass switch 430 and the second light source 304 is lit off.
In FIG. 1B, a first light source 302 and a second light source 304 of a vehicle lamp 300V are configured as a single light emitting unit 301 which is connected to a lighting circuit 400V via a wire harness 305. In FIG. 1A, the wire harnesses 306, 308 respectively include two wires, i.e. four wires in total. In FIG. 1B, the wire harness 305 includes three wires, so that the wires, terminals (pins), and the wire 416 in FIG. 1A can be reduced.
The inventors have recognized the following problems after investigating the lighting circuit 400V of FIG. 1B. Due to poor connection of terminals, wire disconnection or the like, open abnormality may occur in the wire harness 305. Now, it is assumed that open abnormality occurs in a center wire 310 of the wire harness 305.
When open abnormality occurs in the wire 310, the drive current ILAMP flows through the first light source 302 and the second light source 304, and the second light source 304 cannot be turned off, regardless of ON or OFF of the bypass switch 430. It is noted that when open abnormality occurs in the wire 312, problems like those in FIG. 1B would not occur in the vehicle lamp 300U of FIG. 1A since the second light source 304 is turned off
When open abnormality occurs in the bypass switch 430, the second light source 304 cannot be turned off either.
Although the combination of the high beam and the low beam has been described here, the same problem may also occur in combination of other light sources.